Effects of paleogeographic changes and CO2 variability on northern mid-latitudinal temperature gradients in the Cretaceous

The Cretaceous ‘greenhouse’ period (~145 to ~66 million years ago, Ma) in Earth’s history is relatively well documented by multiple paleoproxy records, which indicate that the meridional sea surface temperature (SST) gradient increased (non-monotonically) from the Valanginian (~135 Ma) to the Maastrichtian (~68 Ma). Changes in atmospheric CO2 concentration, solar constant, and paleogeography are the primary drivers of variations in the spatiotemporal distribution of SST. However, the particular contribution of each of these drivers (and their underlying mechanisms) to changes in the SST distribution remains poorly understood. Here we use data from a suite of paleoclimate simulations to compare the relative effects of atmospheric CO2 variability and paleogeographic changes on mid-latitudinal SST gradient through the Cretaceous. Further, we use a fundamental model of wind-driven ocean gyres to quantify how changes in the Northern Hemisphere paleogeography weaken the circulation in subtropical ocean gyres, leading to an increase in extratropical SSTs.

Summary: Gianchandani et al. use an ensemble of HadCM3L simulations to investigate and quantify the relative contributions of atmospheric CO2 and tectonic activity to variations in midlatitude SSTs in the North Pacific Basin during the Cretaceous. The simulations suggest that changes in CO2 and paleogeography had comparable impacts on mid-latitude SST gradients. Gianchandani et al. also highlight the importance of ocean basin aspect ratio in determining gyral circulation, heat transport, and mid-latitude SST patterns in the North Pacific Basin by developing a fundamental model of wind-driven ocean gyres.
• What are the noteworthy results?
The main finding of this study is that changes in CO2 and paleogeography contribute similarly to increasing mid-latitude SST gradients over the Cretaceous. Although it is well established in the literature that changes in CO2 and paleogeography contributed to SST variability over the Cretaceous, this finding is noteworthy because the relative contributions of CO2 and tectonic activity to SST variability are not established. Another noteworthy result is that a decrease in gyral volume transport, and by extension ocean basin aspect ratio, can account for ~80% variability in the meridional SST gradient in the mid-latitude North Pacific over the Cretaceous.
• Will the work be of significance to the field and related fields? How does it compare to the established literature? If the work is not original, please provide relevant references.
Yes, this work is significant for the understanding of ocean circulation and temperature during the Cretaceous. The study outlines a simple model of wind-driven gyres based on the horizontal aspect ratio of the ocean basin that may be applied to other deep-time intervals to better understand the contribution of tectonic activity to changes in ocean circulation and temperature.
• Does the work support the conclusions and claims, or is additional evidence needed?
Overall, yes. However, my main concern is that the range of CO2 captured in these simulations does not capture the inferred paleo-CO2 range of the Cretaceous based on Foster et al. (2017), which could have reached decreased to values of ~280 ppmv during the Late Cretaceous (Lines 116-120). The justification for the CO2 levels of the simulations needs to be explained, particularly because the relationship between CO2 and meridional SST gradient supported by the simulations (at 560-1120 ppmv) is being extended beyond this range of CO2 (down to ~280 ppmv). This directly impacts the how the relationship between CO2 and meridional SST gradient is calculated for the Cretaceous, which is one of the main findings of the study.
• Are there any flaws in the data analysis, interpretation, and conclusions? Do these prohibit publication or require revision?
The statement that atmospheric CO2 fluctuations and paleogeographic changes have a comparable effect on mid-latitudinal SST gradient through the Cretaceous may be too strong of a statement given that the study is restricted to the North Pacific Basin. The authors state that the fundamental model of wind-driven ocean gyres does not apply to the Southern Hemisphere . Some revisions would help broaden the conclusions of the study by investigating the relative importance of CO2 and tectonic activity on SST variability in other ocean basins during the Cretaceous and/or demonstrating how the fundamental model of gyral circulation could also be applied to other time intervals.
• Is the methodology sound? Does the work meet the expected standards in your field?
Overall, yes. However, the methodology requires more justification and explanation CO2 and paleogeographic reconstructions used for the simulations. Both CO2 level and paleogeographic reconstruction are subject to uncertainty that should be more explicitly described.
• Is there enough detail provided in the methods for the work to be reproduced? Yes.
Major comments: • The title reiterates knowledge that is already well established about Cretaceous SSTs and does not highlight the novelty or impact of the study. This title could instead highlight the relative importance of CO2/tectonic activity in the northern mid-latitudes • Main/Introduction: The gap in knowledge motivating this study and broader scientific context from previous literature needs to be explained in the introduction. The first paragraph is already diving into methodology of the study, which has only been motivated by a few sentences. As a result, I am unsure if this study is motivated primarily by a desire to better understand SST variability during the Cretaceous or develop a more general framework for understanding SST variability based on gyral circulation/basin geometry in which the Cretaceous is a case study. After the sentence that motivates looking at other environmental factors (beyond just CO2) (Lines 38-41), there should be a brief discussion of the work that has investigated the impacts of CO2 and paleogeography on ocean temperature and circulation during the Cretaceous to explicitly outline the novelty of the current study (e.g., Ladant et al, 2020 in Climate of the Past; Donnadieu et al., 2016 in Nature Comm).
• Lines 44-45: Why did you choose 560 and 1120 ppmv CO2? This should be justified in the methods section. Although 560 and 1120 ppmv capture most of the inferred paleo-CO2 variability during the Cretaceous, CO2 levels could have been much lower than 560 ppmv during the Late Cretaceous, as you state on Lines 116-120. Regarding Lines 116-120, why do you expect that the relationship between meridional SST gradient and CO2 between two relatively high CO2 levels (560 and 1120 ppmv) would extend to lower CO2?
• Lines 177-179: Although the simple model explains a large proportion of the variance ( Fig. 2A), it shows a roughly linear trend of increasing SST gradient at both CO2 concentrations, and thus does not capture a threshold-like behavior due to paleogeographic changes between ~130-110 Mya, which is very interesting. Before and after this interval, the changes in SST gradient are much smaller. What changes/feedbacks associated with those paleogeographic changes are creating such a dramatic increase in SST gradient during that specific interval?
Minor comments: • Figure 1: I would suggest making red/blue lines and markers thicker to differentiate from black arrows and making black contour labels larger/more visible (I cannot read them) • Line 189-191: Two "results" in the sentence, perhaps break up this sentence with commas or choose a different combination of words here.
• Line 197-200: Citations from previous work should be added here.
• Line 206: Add period to end of sentence.
Reviewer #3 (Remarks to the Author): Please find my comments in the attached word file.
Reviewer #3 Attachment on the following page Review: "Both tectonic activity and CO2 variability affect temperature gradients in the Cretaceous" Kaushal Gianchandani, Sagi Maor, Ori Adam, Alexander Farnsworth, Hezi Gildor, Daniel J. Lunt, Nathan Paldor Gianchandani et al. present a study that uses general circulation modeling to investigate the driving mechanisms of changes in the Cretaceous meridional temperature. The authors conclude that both paleogeographic changes and decreasing pCO2 contributed equally to a decrease in the midlatitude temperature gradient of about 6 °C from the Early to Late Cretaceous (i.e., from the Valanginian to the Maastrichtian). This change in temperature gradient was caused by a decrease in oceanic heat transport resulting from a weakening of the wind-driven gyre circulation in the northern Paleo-Pacific in response to a decreased latitudinal extent of the Pacific basin.
As someone who has worked a lot on Cretaceous paleoceanography and paleoclimatology, but is not an expert in general circulation modeling, I find the manuscript to be well written, well structured, and easy to follow. The science is sound and the interpretations are supported by the data. The conclusions are presented in a clear manner and are relevant to the broader geoscientific community. As such, I consider the manuscript to be well suited for publication in Nature Communications and have only a few minor comments.
1.) Paleogeography: I recommend including some background information on the paleogeography implemented in the model in the Methods section. Because paleogeographic restorations of the Paleo-Pacific are complicated by extensive subduction of oceanic crust since the Cretaceous, it would be particularly helpful to include a brief comparison between the implemented paleogeography and other published plate tectonic models to show that the general trends in LX and LY are consistent across the various available models. 2.) Role of sea-level change: Do the changes in land-sea distribution shown in Fig. S1 and Fig. S6 reflect solely plate tectonic processes (i.e., changes in the distribution of oceanic and continental crust) or also global sea-level changes (i.e., flooding of continents)? If both processes are involved, the authors may consider replacing "tectonic activity" with "paleogeographic changes" in the title of the manuscript. 3.) Paleo-SST data: I suggest to include a graphical comparison of proxy-based SST estimates and model-derived latitudinal temperature gradients, at least for the Valanginian and Maastrichtian, to demonstrate that the model is capable of capturing the general patterns evident in the proxy data.

Re: Point-by-_^X]c aTb_^]bT c^aTeXTfTabs R^\\T]cb
Reviewer #1: This paper uses a suite of climate model simulations to investigate the Cretaceous greenhouse period. In particular, the authors study modelled midlatitude sea-surface temperature (SST) gradients in the Northern hemisphere and the respective roles of changes in continental configuration, atmospheric carbon dioxide (CO2), and meridional heat transport in the wind-driven ocean gyres. The novel aspect of this work consists of an analysis of oceanic heat transport relating to the changing aspect ratio of the paleo-Pacific ocean basin.
We thank the reviewer for her/his positive remarks on our manuscript and appreciate the suggestion to analyze the datasets provided by Li et al. (2022) and Landwehrs et al. (2021). We summarize the findings from our examination of the two datasets below.
This is certainly an interesting paper which will be useful to the paleoclimate community. My main recommendation would be not to focus on the Hadley model alone, but to include existing ( The ensemble of simulations discussed in Landwehrs et al. (2021) shows a decrease in mid-latitudinal SST gradient during the Cretaceous period, which is in contrast with what the paleotemperature proxies suggest %Es8aXT] Tc P[)' -+,2&. Furthermore, the fast statistical-dynamical atmospheric model in CLIMBER-3z employed for these simulations has a rather coarse resolution of 7.5° in latitude (compared with 2.5° in the HadCM3L model). Thus, it does not capture the subtle variations in the curl of the surface wind-stress induced by paleogeographic changes and the corresponding changes in the distance between wind-stress extrema (Ly) on geological timescales, which underpin our analysis.
Given these constraints, we are unable to extend our analysis to other ensembles of paleo-climate simulations at this stage. Nonetheless, we are grateful for the reviewerss suggestion.
We o NYRe RcV eYV _`eVh`ceYj cVdf]ed8 The main finding of this study is that changes in CO2 and paleogeography contribute similarly to increasing mid-latitude SST gradients over the Cretaceous. Although it is well established in the literature that changes in CO2 and paleogeography contributed to SST variability over the Cretaceous, this finding is noteworthy because the relative contributions of CO2 and tectonic activity to SST variability are not established. Another noteworthy result is that a decrease in gyral volume transport, and by extension ocean basin aspect ratio, can account for ~80% variability in the meridional SST gradient in the midlatitude North Pacific over the Cretaceous. Overall, yes. However, the methodology requires more justification and explanation of CO2 and paleogeographic reconstructions used for the simulations. Both CO2 level and paleogeographic reconstruction are subject to uncertainty that should be more explicitly described.
We thank the reviewer for the overall positive comments and for providing us some critical feedback which has helped us improve the manuscript. We provide a point-bypoint response to her/his comments below. o DZ_Vd -33-179: Although the simple model explains a large proportion of the variance ( Fig. 2A), it shows a roughly linear trend of increasing SST gradient at both CO2 concentrations, and thus does not capture a threshold-like behavior due to paleogeographic changes between ~130-110 Mya, which is very interesting. Before and after this interval, the changes in SST gradient are much smaller. What changes/feedbacks associated with those paleogeographic changes are creating such a dramatic increase in SST gradient during that specific interval?
We agree that the steep increase in temperature gradients between 130 and 100 Ma is an interesting feature. In the revised version, we highlight this and acknowledge that it lies outside the range in which our best-fit curve is expected to vary. The steep increase stems from the abrupt decrease in SST at 50°N (Fig. S5, Revised Manuscript). This can potentially be related to vanishing of the sub-polar gyre from the Barremian to the Albian and the entrainment of high-latitude cold water by the midlatitudinal gyre ( Fig. R1A-C). In the present-day Atlantic, there is clear demarcation between the mid-latitudinal and the subpolar gyres, which limits the mixing of water at the poleward edge (Equatorward edge) of the mid-latitudinal (subpolar) gyre. However, other factors including paleogeography driven polar amplification could also contribute to the cooling effect. Examining the precise climatic variations/feedbacks associated with paleogeographic changes that lead to this abrupt cooling is beyond the scope of the present short manuscript. This is discussed in Lines 240 -248.

Reviewer #3:
Gianchandani et al. present a study that uses general circulation modeling to investigate the driving mechanisms of changes in the Cretaceous meridional temperature. The authors conclude that both paleogeographic changes and decreasing pCO2 contributed equally to a decrease in the midlatitude temperature gradient of about 6 °C from the Early to Late Cretaceous (i.e., from the Valanginian to the Maastrichtian). This change in temperature gradient was caused by a decrease in oceanic heat transport resulting from a weakening of the wind-driven gyre circulation in the northern Paleo-Pacific in response to a decreased latitudinal extent of the Pacific basin. As someone who has worked a lot on Cretaceous paleoceanography and paleoclimatology, but is not an expert in general circulation modeling, I find the manuscript to be well written, well structured, and easy to follow. The science is sound and the interpretations are supported by the data. The conclusions are presented in a clear manner and are relevant to the broader geoscientific community. As such, I consider the manuscript to be well suited for publication in Nature Communications and have only a few minor comments.
We thank the reviewer for the encouraging remarks and for providing us critical feedback which has helped us improve the manuscript. We provide a point-by-point response below.
1.) Paleogeography: I recommend including some background information on the paleogeography implemented in the model in the Methods section. Because paleogeographic restorations of the Paleo-Pacific are complicated by extensive subduction of oceanic crust since the Cretaceous, it would be particularly helpful to include a brief comparison between the implemented paleogeography and other published plate tectonic models to show that the general trends in LX and LY are consistent across the various available models.
?] PRR^aSP]RT fXcW cWT aTeXTfTasb suggestion, we have further elaborated on the paleogeography implemented in the HadCM3L model in the Data Availability section of the revised manuscript (Lines 441-449). We agree that a comparison between Lx and Ly as obtained from multiple models will strengthen our sermon and we attempt to do the same by examining the publicly available data provided by Li et al. (2022) and Landwehrs et al. (2021). However, we are unable to present a comparison at this stage for the following reasons: a) In the simulations discussed in Li et al. (2022), both the atmospheric CO2 concentration and the paleogeography change simultaneously which makes it difficult to disentangle the respective contribution of each of these components to the changes in the curl of the surface wind-stress and the corresponding changes in the distance between wind-stress extrema (Ly).
b) The atmospheric component of the climate model (CLIMBER-3z) employed for the simulations discussed in Landwehrs et al. (2021) has a rather coarse resolution of 7.5° in latitude. Thus, it does not resolve the paleogeography driven changes in the meridional extent of the Hadley cell or the subtle variations in the surface winds which is critical for calculating the changes in the gyral circulation and therefore the oceanic heat transport.
We note that our approach for quantifying the effect of aspect ratio ( L y L x ) on the intensity of gyral circulation was applied in a previous study to the present-day ocean gyres in different ocean basins (Gianchandani et al., 2021). We showed that the small volumetric (mass) transport of the East Australian Current compared to other Western Boundary Currents can be attributed to the geometry of the South Pacific Basin, i.e., 7dbcaP[XPsb ;PbcTa] coastline is not long enough to support a strong WBC in the zonally wide South Pacific Ocean.
2.) Role of sea-level change: Do the changes in land-sea distribution shown in Fig. S1 and Fig. S6 reflect solely plate tectonic processes (i.e., changes in the distribution of oceanic and continental crust) or also global sea-level changes (i.e., flooding of continents)? If both processes are involved, the authors may consider replacing "tectonic activity" with "paleogeographic changes" in the title of the manuscript.
The change in land-sea distribution does include a sea level component and in general our chosen paleogeographic represent the high-stands in sea level. Thus, we thank the reviewer for pointing this out accept the suggestion to replace tectonic activity with paleogeographic changes in the title and. The modified title now is: pEffects of paleogeographic changes and CO2 variability on northern mid-latitudinal temperature gradients in the Cretaceous)q 3.) Paleo-SST data: I suggest to include a graphical comparison of proxy-based SST estimates and model-derived latitudinal temperature gradients, at least for the Valanginian and Maastrichtian, to demonstrate that the model is capable of capturing the general patterns evident in the proxy data.
We do not compare the model-derived temperature gradients with proxy data since the atmospheric CO2 and continental arrangement do not coevolve in the current set of simulations. These simulations are not designed to represent actual changes in climate, but instead are idealized with constant CO2 concentrations through time. >^fTeTa' fT PVaTT fXcW cWT aTeXTfTasb bdVVTbcX^] cWPc Xc Xb X\_^acP]c c^QT]RW\PaZ model data against geochemical proxies and plan to address it in a future work using data from multiple climate models in which atmospheric CO2 and geography are varied simultaneously (Lines 279-286).
4.) Figure 1: The labeling of the contour lines in Figures 1A and 1B is barely visible.
The suggestion was incorporated and the figure was modified accordingly.
Line 138: I am aware that this change is due to a suggestion of Reviewer 3, but "global zonally averaged SST" sounds weird. Maybe rather specify that the average was computed "over all longitudes" or similar?
Reviewer #2 (Remarks to the Author): I appreciate the work that the Gianchandani et al. have accomplished to better motivate and expand the impact of the study. Specifically, the extension of the gyral circulation model from the Cretaceous to the Paleogene is a great addition. However, much of the manuscript (title, abstract, introduction, methods, some results, and discussion) have not all been equally updated to reflect the inclusion of the Paleogene and how this alters/broadens the impact and conclusions of the study. I have included more detailed suggestions in the comments below.
Major Comments • Introduction: You have provided the background to explain how your study advances our understanding of Cretaceous SSTs and ocean circulation, but you have expanded the scope to include the Paleogene which is not mentioned in the introduction. Additionally, you have not fully incorporated the paleoclimate and gyre circulation components of the introduction. One suggestion to incorporate Cretaceous-Paleogene climate with your gyre model in the introduction is to introduce (1) description of meridional SST patterns in the Cretaceous-Paleogene, (2) problem: quantifying the impact of paleogeography on these SST patterns is important but complicated as previous work has shown, (3) gap: surface gyres are an important driver of SST patterns and have not been directly linked to paleogeographic changes in previous work, (4) solution: Stommel's simple model of gyral circulation can be used to quantify the influence of paleogeography on past SST patterns, and thus better understand the relative importance of other climate forcings like CO2. I would also introduce all of these general ideas before including any details about your simulations (lines 53-56).
• The Results section, including "Wind-driven gyral circulation in the Cretaceous" should include a justification of why Stommel's model also works for the Paleogene • The Results section, "Variability in meridional SST gradients and its dependence on atmospheric CO2 and paleogeography", does not touch on the differences in long-term trends of meridional SST gradients during the Paleogene compared to the Cretaceous and this is a new interesting component of the study. • Line 188: I think this should be modified to "compared to long-term trends in pCO2". I think you must be careful with these statements because you are not capturing the entire inferred range of CO2 during this Cretaceous-Paleogene (you are capturing the range of the smoothed long-term trend). You may even add a sentence that states that CO2 may have a larger impact if shorterterm fluctuations in atmospheric CO2 and/or if low CO2 estimates from some proxies (line 102-103) are accounted for.
• Lines 253-258: How does this logic extend from the Cretaceous through the Paleogene? My understanding is that Figure S7 demonstrates that land area fraction continues to increase through the Paleocene and Eocene, but in Figure S6 there is not a concurrent decrease in Ly with a decrease geographical extent of the ocean basin during this time. The relationship between Ly and maximal volume transport with SST gradient seems to hold from the Cretaceous-Paleogene, but this does not track with the geographical extent of the ocean basin during the Paleogene. If I understand that correctly, is there something other than paleogeography that is driving an increase in Ly during the Paleogene?
• Lines 435-454: I think much of these simulation details should be moved out of the Data Availability statement into the the first paragraph of the Results "Numerical Simulations" to outline basic structure/motivation of simulations used and details should be placed in a Methods section at the end. I'm thinking of an organization like Sauermilch et al. (2021) in Nature Comms https://doi.org/10.1038/s41467-021-26658-1 Minor Comments • Line 189: I would add a topic sentence to this paragraph saying that differences in long-term SST gradients between the simulations and paleotemperature reconstructions may be attributed to the limited spatial distribution of available proxies (the topic of this paragraph is unclear from the first sentence so the transition from the last paragraph to this one is difficult to understand) • Lines 234-239: Split this into two sentences at "which"